ASSESSMENT OF COASTAL HAZARD RISK ZONES FOR THE SOUTHERN POVERTY BAY AREA

Transcription

1 ASSESSMENT OF COASTAL HAZARD RISK ZONES FOR THE SOUTHERN POVERTY BAY AREA Report prepared for Gisborne District Council JUNE 2004 C.R. 2004/1 Jeremy G Gibb 2009 All rights reserved. This work is entitled to the full protection given by the Copyright Act 1994 to the author. No part of this work covered by the author's copyright may be reproduced or copied in any form or by any means (graphic, electronic or mechanical, including photocopying, recording, recording taping, or information retrieval systems) without the written permission of the author. It is accepted that the client is able to copy any report in its entirety for internal purposes and distribution to its consultants.

2 Preliminary part DISCLAIMER Coastal Management Consultancy Limited (the Service Provider) shall have no liability; i. to any person other than the client to whom the Service Provider s report is addressed; nor, ii. in the event that the Service Provider s report is used for any purpose other than the specific purpose stated in the report. Coastal Management Consultancy Ltd (CMCL) C.R. 2004/1 II

3 Preliminary part TABLE OF CONTENTS 1. INTRODUCTION THE PURPOSE OF COASTAL HAZARD ZONES NATURAL COASTAL HAZARDS PREVIOUS HAZARD ASSESSMENTS Orongo Beach Young Nicks Head Muriwai Beach METHODS EROSION LANDSLIP FLOODING GEOLOGY AND HAZARDS THE COASTAL PLAIN Shoreline Movements at Muriwai Beach Shoreline Movements at Orongo Beach YOUNG NICKS HEAD Shoreline Movements around Young Nicks Head FLOOD HAZARD Wave Storms Tsunami COASTAL HAZARD ZONE (CHZ) ASSESSMENT CLHZ ASSESSMENT CLHZ Widths CEHZ ASSESSMENT Quantifying the Factors CEHZ Widths...16 Coastal Management Consultancy Ltd (CMCL) C.R. 2004/1 III

4 Preliminary part 5.3. CFHZ ASSESSMENT Flood Velocity Zone Flood Ponding Zone CFHZ SUMMARY AND CONCLUSIONS RECOMMENDATIONS REFERENCES...19 TABLES Table 1: Aerial Surveys of the Muriwai Beach Young Nicks Head area. Survey Data Source: NZ Aerial Mapping Ltd. Contact print scale determined by CMCL. Coverage refers to the study area. 5 Table 2: Net rates of erosion of Muriwai Beach at the beach end of Muriwai Beach Road, measured from both the contact prints listed in Table 1 and the GIS plans of GDC. 8 Table 3: Rate and direction of migration of the true right bank of the Waipaoa River mouth between 1899 and 2002 (103 years) relative to an extended line from the 1948 East end of Muriwai Beach Road. Data measured from vertical aerial photographs and from GIS maps of historic shoreline positions compiled by GDC...8 FIGURES Figure 1: Unrectified vertical aerial photograph (SN25046) of the study area (rectangle) taken 23 December Source; NZ Aerial Mapping Ltd, Hastings...2 Coastal Management Consultancy Ltd (CMCL) C.R. 2004/1 IV

5 ASSESSMENT OF COASTAL HAZARD RISK ZONES FOR THE SOUTHERN POVERTY BAY AREA by Jeremy G Gibb, PhD, BSc (Hons), TIPENZ Director Coastal Management Consultancy Limited, Tauranga, New Zealand 1. INTRODUCTION In October 2003, Coastal Management Consultancy Ltd (CMCL) were commissioned by Gisborne District Council (GDC) to undertake a Coastal Hazard Risk Zone assessment for the Southern Poverty Bay area from the Waipaoa River mouth to the South end of Orongo Beach, including Young Nicks Head (the study area) (Figure 1). The assessment was to be essentially a desktop exercise by CMCL at their office in the Western Bay of Plenty. The desktop exercise was to be based on information supplied mostly by GDC, including interviews with local residents about past hazard events, supplemented by relevant information held by CMCL from previous work. The terms of reference for the study required that the Coastal Hazard Risk Zones replicate those previously assessed for the Wainui Beach area (CMCL 1995; 2001). In line with this, Coastal Erosion Hazard Zones (CEHZ) were to be assessed for both Muriwai and Orongo Beaches and Coastal Landslip Hazard Zones (CLHZ) for Young Nicks Head. CMCL were to plot the Risk Zones on rectified Orthophotomaps supplied by GDC for digitising and inclusion in to their computer based Geographic Information System (GIS). In addition, CMCL were to contribute to defining Coastal Flood Hazard Zones (CFHZ) with David Peacock, GDC, for the low-lying coastal plain THE PURPOSE OF COASTAL HAZARD ZONES Coastal Hazard Zones (CHZ) identify land that is, or is likely to be subject to the effects of coastal hazards over a defined period. For New Zealand, that period has been standardised as 100 years since its endorsement by the National Water and Soil Conservation Authority in March CHZs are used to manage subdivision, use and development along the coast. To facilitate such management, CHZs are often subdivided into Coastal Hazard Risk Zones (CHRZ) grading from Extreme next to the sea, to High and Moderate inland. Risk zones provide Local Authorities with opportunities to adopt risk avoidance measures through appropriate policies and rules in their District Plans and Regional Coastal Environment Plans. Coastal Management Consultancy Ltd (CMCL) C.R.2004/1 1

6 Figure 1: Unrectified vertical aerial photograph (SN25046) of the study area (rectangle) taken 23 December Source; NZ Aerial Mapping Ltd, Hastings. Coastal Management Consultancy Ltd (CMCL) C.R.2004/1 2

7 1.2. NATURAL COASTAL HAZARDS A natural hazard is defined by Varnes (1984) as the probability of occurrence within a specified period of time and within a given area of a potentially damaging natural phenomenon. According to S.2 (RMA-91), such phenomena include; any atmospheric or earth or water related occurrence (including earthquake, tsunami, erosion, volcanic and geothermal activity, landslip, subsidence, sedimentation, wind, drought, fire, or flooding) the action of which adversely affects or may adversely affect human life, property, or other aspects of the environment. Although the term adverse effects is not precisely defined in the RMA-91, under S.3 the term may mean; any temporary or permanent effect; any past, present, or future effect; any cumulative effect which arises over time or in combination with other effects regardless of the scale, intensity, duration or frequency of the effect. Any potential effect of high probability and any potential effect of low probability which has a high potential impact. 2. PREVIOUS HAZARD ASSESSMENTS In 1994, CMCL assessed Areas Sensitive to Coastal Hazards (ASCH) for selected parts of the Gisborne District coast including the study area (CMCL 1994). For the 1994 study, 112 Stations were surveyed in September and November 1994 including 7 in the study area. For each station, a comprehensive Coastal Hazards Database was established, based on standardised techniques which classified the coast in terms of its relative sensitivity to coastal hazards (Gibb et al 1992). The major natural coastal hazards identified in the study area and indeed, the entire Gisborne District, were erosion, landslip and flooding (CMCL 1994) Orongo Beach The 2 metre-high sand barrier was found to have a long-term trend of duneline retreat at m/year ( ) with maximum short-term duneline fluctuations of 20 to 30 m. The relatively low sand barrier was also found to be subject to Storm Wave Runup (SWRU) of approximately 3-4 m above Mean High Water Springs (MHWS) and tsunami runup from the 1947 locally generated event of 5-6 m a. MHWS which would easily overtop the barrier flooding the small valley behind (CMCL 1994). The 1994 study surmised that there was a high probability that the sea will breach Orongo Beach this century eventually isolating Young Nicks Head as an island which would have significant effects on the present orientation of the Poverty Bay shoreline. Orongo Beach was classified as having a Very High Coastal Sensitivity Index and an ASCH width of 200 m, extending up to 1,500 m inland to principally allow for inundation once the low dune barrier is permanently breached (CMCL 1994) Young Nicks Head Seacliffs cut into the Late Tertiary sandstone-mudstone sedimentary lithologies were found to be retreating at to m/year, averaging m/year. The study Coastal Management Consultancy Ltd (CMCL) C.R.2004/1 3

8 noted that persistent coastal erosion is continually removing the lateral support of the sedimentary rocks so that old landslides are reactivated and new ones occur, the type and magnitude of landslide depending on the structure and composition of the lithology and elevation of the seacliffs. For Young Nicks Head, the occurrence of both rockfalls and slumps was recorded. The area was classified as having a Medium Coastal Sensitivity Index with ASCH widths ranging from 100 m up to 530 m (CMCL 1994) Muriwai Beach The 0.7 to 1.3 m-high sand barrier was found to have a long-term trend of duneline advance from 0.39 m/year up to 2.12 m/year, generally averaging 0.83 m/year, over survey periods ranging from 51 to 107 years. Short-term duneline fluctuations ranged from m up to m. In addition, the low-lying barrier was found to be subject to SWRU of 3-4 m a. MHWS and tsunami runup from the 1947 event of 5-6 m a. MHWS (CMCL 1994). The natural coastal hazards recorded included erosion from short-term duneline fluctuations and migration of the Waipaoa River mouth and flooding from both the sea and Waipaoa River. The Muriwai Beach area was classified as having a High to Very High Coastal Sensitivity Index with ASCH widths ranging from 180m up to 2,500 m (CMCL 1994). It was noted that the land is undergoing tectonic downdrop and in some places is either at MSL or less than 2 m above. The inland boundary of the ASCH was defined by the 3 m contour on the basis of maximum SWRU and tsunami runup levels, sealevel rise, and a gradient of less than one degree of the coastal hinterland. The relatively large area covered by the ASCH mostly reflected the effects of flooding from the sea (CMCL 1994). 3. METHODS Based on the previous work outlined in Section 2 of this report (CMCL 1994; 1995; 2001) the following methods were adopted for this assessment to quantify the identified coastal hazards EROSION For the study area, coastal erosion and accretion rates were quantified for 50 Stations between Orongo Beach and the Waipaoa River mouth over a coastline distance of km. Both short-term and long-term rates were quantified at each Station by measuring offsets between historic shoreline positions. The historic shoreline positions were plotted by the GIS Section of GDC from Aerial Plan 1429, Sheets 8, 9 and 10 at 1:5000 Scale. With the exception of Young Nicks Head, the historic shorelines included a recent coastal survey made by both Ian Hughes, Surveyor, GDC, and CMCL on 29 May The GIS Section reproduced the digitised historic shoreline positions on fully rectified Orthophotomaps at 1:2,500 and 1:5,000 Scales for Orongo Beach-Young Nicks Head, and Muriwai Beach, respectively. Rates for each of the 50 Stations are provided in Table A-1 in Appendix A. 4

9 3.2. LANDSLIP For Young Nicks Head the extent and types of landslides were identified both on the ground in May 2002, from contact prints of the 8 vertical aerial surveys listed in Table 1 that span a 62 year-period ( ), and on the Orthophotomaps at 1:2,500 and 1:5,000 Scales supplied by GDC. Information on historic landslip events was also gathered by GDC staff for this study from interviews with long-standing local reliable observers and early reports. Table 1: Aerial Surveys of the Muriwai Beach Young Nicks Head area. Survey Data Source: NZ Aerial Mapping Ltd. Contact print scale determined by CMCL. Coverage refers to the study area. SURVEY NUMBER SURVEY DATE SURVEY RUNS CONTACT PRINT SCALE COVERAGE Feb-38 F/1 to 4 1:11,600 Total G/1 to Apr /66 to 69 1:23,580 Total 1101/67 to Jul-48 F/31 to 39 1:10,450 Part - Muriwai Beach only Aug /6 to 8 c.1:10,000 Part - Waipaoa River mouth only Nov /54 to 56 1:26,365 Total 3807/54 to Aug-75 A/28 to 29 1:26,975 Total 9-Sep-75 B/27 to 30 C/28 to Jan-79 I/1 to 2 1:48,745 Total Dec-00 C/153 to 158 1:50,000 Total 3.3. FLOODING For the flood prone low-lying coastal plain adjacent to both Muriwai and Orongo Beaches, information on past flood events was gathered by GDC staff for this study from interviews with long-standing, reliable local observers and early reports. From this information the heights above Mean Sea Level (MSL) Gisborne Datum of past flood levels were fixed by standard survey methods. To assess sea flood levels this century, the combined effects of forecast local relative sea level rise (RSLR), storm wave runup (SWRU), and tsunami wave runup (TWRU) were also considered. 4. GEOLOGY AND HAZARDS For the purposes of this study, the Southern Poverty Bay area is composed of the following two interconnected parts: i. Muriwai coastal plain area 5

10 ii. Young Nicks Head area The Muriwai coastal plain area includes both Muriwai and Orongo Beaches and the coastal plain connecting them. The area is bounded by the Waipaoa River to the North; State Highway 2 to the West; the coastline and seabed of the coastal plain and landward base of Young Nicks Head area to the East; the 5-metre contour and the Southern end of Orongo Beach to the South. The Young Nicks Head area includes the seacliffs and seabed surrounding the headland to the North, East and South, and the intersection of the West and landward part of the headland with the coastal plain as delineated by the 2-metre contour, and the hills comprising Young Nicks Head THE COASTAL PLAIN The Muriwai coastal plain is composed of a mix of Late Holocene beach ridges, sand dunes and sand spits, alluvial flats, wetlands, the Wherowhero Estuary, drains and creeks and the lower reaches and mouth of the Waipaoa River. The 1.7 to 2.3 kilometre-wide coastal plain is bounded to seaward by Muriwai Spit formed from Medium to Fine Sand supplied by the Waipaoa River. According to Brown (1995), the Poverty Bay flats overly an actively downwarping syncline or structural depression. Radiocarbon dates suggest that the Muriwai coastal plain formed over the last 2,000 to 5,000 years during the Late Holocene Epoch. At the end of Beach Road, the 2000-year shoreline is about 700 m from the present coast, suggesting a Late Holocene rate of shoreline advance of 0.35 m/year. There is compelling evidence in Brown (1995) for a trend of tectonic downdrop of the Muriwai plains over at least the last 5,000 years. Reliable dates and levels from Borehole PN15 (Grid. Ref Y18/379646) suggest downdrop rates of 2.0 to 2.5 m/1,000 years (2-2.5 mm/year). It is interesting to observe that during the Late Holocene, sedimentation rates generally kept pace with tectonic downdrop thus maintaining the plains surface at or a few metres above MSL. In 1938, well-defined sets of beach ridges extended up to about 930 m inland from Muriwai Beach. The sets of ridges are formed from 2 components. The 230 metre to 465 metre-wide landward set in 1938 were bounded by wetlands of similar origin to the present-day Wherowhero Estuary. The orientation of the ancient ridges suggests formation from a net Northerly longshore drift. The average heights of the ridges was about 2.3 m MSL. The landward beach ridges formed seaward of the Southerly offset Waipaoa River channel at the time. The 395 metre to 465 metre-wide seaward set in 1938 were bounded by a wetland and a 2,150 metre-long Muriwai Beach. The innermost beach ridge is of similar morphology to the present day barrier sand spit separating Wherowhero Estuary from the sea, suggesting that it formed from a net Southerly longshore drift. Seaward of this ancient sand spit were coalesced sets of beach ridges that had built out from a plentiful supply of sand seaward of the southerly offset Waipaoa River channel at the time. The average height of the seaward set of ridges is presently unknown. Landward and North of the beach ridges was the Wherowhero Creek and a wetland connected to the Creek. In the 1930s and 1940s the wetlands and Creek traced a path sub-parallel to the present day Wherowhero Estuary, once a Southerly migratory arm of the Waipaoa River. The inner wetland landward of the beach ridges is likely to 6

11 have formed when the Waipaoa River discharged to the sea at firstly, Orongo Beach and secondly, at the landward base of Young Nicks Head. From at least 1938 to the mid 1970s, Orongo Beach was open to the sea. Behind the beach a tidal estuary existed extending to the base of the hills on each side and as far West as the Muriwai coastal plain. The estuarine nature of this landform is interpreted here as being a remnant river course and an ancient mouth for the Waipaoa River. In the 1960s the mouth of the estuary was closed by a fan of beach sand driven into the mouth by heavy waves either from storms or the 1961 Chilean tsunami. Since 1964, the Orongo Beach estuary has remained closed to the sea. The present Muriwai village is located on a smoothed low-lying spur emanating from the foothills behind. The spur is about 4.5 to 6.5 m above MSL. The land surrounding the spur falls away to less than 2 m a.msl to the East and North and less than 4 m a.msl to the South. The steep faceted face on the spur just West of Muriwai Settlement suggests that it was originally cut by the sea about 4,000 to 6,500 years ago when sea-level in the New Zealand region was close to the present level (Gibb 1986). The base of the ancient seacliff is about 7 m a.msl. The Muriwai spur is interpreted here as being a remnant promontory now stranded by accretion that once extended into a shallow embayment. The original Wherowhero Creek wetland shown on the 1930s and 1940s aerial photographs extended most of the way to the landward base of Young Nicks Head. The dimensions of this wetland are similar to those of the lower reaches of the Waipaoa River suggesting that it was once the lower reach of the river and that the river discharged at the landward base of Young Nicks Head Shoreline Movements at Muriwai Beach Tables 2 and 3 summarise dynamic changes to Muriwai Beach as a direct consequence of the very unstable, migratory behaviour of the Waipaoa River mouth. Prior to 1841 the river drained through the Awapuni Lagoon and discharged East of the Gisborne abattoirs. Sometime after 1841, the mouth moved to its present position. The most rapid changes observed occurred between 1938 and 1945 when Muriwai Beach retreated 325 m at m/year at the end of Muriwai Beach Road (Table 2). As a consequence, most of the seaward set of beach ridges was destroyed along with farmland and possibly 10 buildings evident on the 1938 photograph comprising Muriwai Beach Town ( Piiti Taone ). 7

12 Table 2: Net rates of erosion of Muriwai Beach at the beach end of Muriwai Beach Road, measured from both the contact prints listed in Table 1 and the GIS plans of GDC. SURVEY YEAR MURIWAI BEACH NET CHANGE RATE (m) (m) (m/y) The extreme rate of erosion was directly the result of the Southward migration of the Waipaoa river mouth. From 1938 to 1945 the mouth migrated 1,680 m South at 240 m/year (Table 3). As in the past the mouth was once again located at the landward base of Young Nicks Head and the river flowed out along the Northwestern edge of the headland into Poverty Bay. Table 3: Rate and direction of migration of the true right bank of the Waipaoa River mouth between 1899 and 2002 (103 years) relative to an extended line from the 1948 East end of Muriwai Beach Road. Data measured from vertical aerial photographs and from GIS maps of historic shoreline positions compiled by GDC. SURVEY YEAR PERIOD DISTANCE NORTH (N) OR SOUTH (S) OF MURIWAI BEACH RD MIGRATION NORTH (N) OR SOUTH (S) RATE (years) (m (m) (m/year) ,920 N ,320 N 600 S N 1,400 S S 1,680 S ,620 N 3,380 N Artificial cut ,720 N 100 N ,300 N 420 N ,800 N 500 N ,900 N 100 N ,800 N 100 S ,700 N 100 S ,120 N 580 S ,720 N 400 S

13 In early 1945, a timber tressle and rock groyne was constructed close to the present mouth and a cut made through the sand barrier spit both of which are evident on the 1945 photograph. By June 1946, the river was discharging through the cut and the lower reach was cut off from the river as the sand barrier folded in at this point welding on to the land at the end of Beach Road. The cutting off of the lower reach of the Waipaoa River from the river resulted in the formation of the tidal Wherowhero Estuary. The erosion between 1938 and 1946 also rejuvenated the old river meander channel to the West of the 1946 channel that had since become a wetland, re-connecting it to the cut-off lower reach of the Waipaoa River. As a result the Wherowhero Creek now discharged directly into the newly formed Wherowhero Estuary which has remained open to the sea near the base of Young Nicks Head since the 1940s. Since the Waipaoa River mouth was stabilized in 1946 it has continued to migrate both North and South at rates of 11.1 to 200 m/year (Table 3). From 1948 to 1979, the mouth migrated about 1,280 m North at 41 m/year. From 1979 to 2002, the migration direction suddenly reversed and the mouth migrated 1,180 m South at 51 m/year largely as a result of Tropical Cyclone Bola (March 1988) breaching the spit. When inspected during fieldwork in May 2002, the Waipaoa River mouth was still migrating South at a rapid rate of 200 m/year as it did prior to 1946 (Table 3). In August 2002 the mouth was again diverted straight out (David Peacock, pers. comm. 2004). Notwithstanding the changes from migration of the Waipaoa River mouth the open-coast beach has generally advanced from accretion at 0.16 to 0.74 m/year since the early 1900s. The accretion is directly from Fine Sand supplied by the Waipaoa River mostly during floods. The trend of duneline advance has been punctuated by short-term duneline fluctuations of the order of 60 to 70 m (Table A-1, Appendix A). The small beach at the base of Young Nicks Head to the East of Wherowhero Estuary has retreated at to m/year since 1938 and appears to be a very temporary landform that is directly affected by the unstable entrance to Wherowhero Estuary. Prior to 1938, the beach was advancing at 2.04 to 3.27 m/year (Table A-1, Appendix A) Shoreline Movements at Orongo Beach The Late Tertiary mudstone headlands adjacent to Orongo Beach have retreated at to m/year last century (Table A-1, Appendix A). In contrast, the sand dunes have advanced from accretion at 0.08 to 0.27 m/year during the same period. The accretion has largely occurred since closure of the small estuary opening to the sea some time after The golden beach sand is of similar origins to Wainui Beach and has been supplied from the combination of volcanic minerals from the Taupo Volcanic Zone and broken shell. The present accreting landform is a relatively low dune barrier that is still subject to occasional wave overtopping during severe wave storms and tsunami. 9

14 4.2. YOUNG NICKS HEAD Young Nicks Head has been sculptured out of Late Tertiary undifferentiated massive and bedded mudstone with intercalated beds of sandstone, conglomerate and tuff. The sediments were originally laid down within a marine environment about 11.2 to 5.3 million years ago. Since then, they have been uplifted and deformed as a consequence of tectonic movements and large earthquakes associated with continuous pressure movements at about 47 mm/year on the plate boundary between the Pacific Plate to the East and the Australian Plate to the West. The plate boundary lies along the axis of the Hikurangi Trough located about 90 km offshore (Mazengarb & Speden 2000). The aerial surveys covering young Nicks Head (Table 1) reveal that the seacliffs have been subject to landslip both prior to and after 1938 up to at least The extent and type of landslips are different on the South coast compared to the North coast. On the South coast there are relatively massive slumps the largest of which measures about 800 m long by up to about 300 m deep and which occurred sometime prior to It may have been triggered by the 1931 Magnitude 7.8 Hawke s Bay Earthquake. Elsewhere, there are relatively smaller scale but similar type of slumps along the South coast. In contrast, no such slumps are visible on the North coast. Instead the type of landslip appears to be mostly localised relatively small-scale rock falls that create temporary debris fans on the beach. The fan material does not last and is destroyed relatively quickly by the sea. The sandstone component forms Very Fine Sand which is added to the beach and the mudstone component, Mud which is carried in suspension by currents offshore to be deposited on the continental shelf. The pattern and types of landslips around Young Nicks Head parallel those around Tuahine Point at the Northern end of Poverty Bay. At Tuahine Point the strata dip East at about 20 degrees. On the East side of the Point there are relatively large scale bedding plane slumps. On the West side there are rock falls, both types of landslip coupled with cliffbase wave erosion leading to seacliff retreat at about m/year (CMCL 1995, 2001). At Young Nicks Head, the strata dip Southeast at about 10 degrees (Mazengarb & Speden 2000). Comparing the two areas suggests that the large-scale slips on the South side of Young Nicks Head are bedding plan failures similar to Tuahine Point. On the North side, the observed rockfalls and steep faceted cliffs parallel those on the Sponge Bay side of Tuahine Point. The strata dip into the cliffs in both places Shoreline Movements around Young Nicks Head Since 1938, the cliffline on the South side of Young Nicks Head has retreated at to m/year, generally averaging about m/year. Adjacent to the bedding plane landslips the cliffline has temporarily advanced 15 to 82 m before being worn back by the sea (Table A-1, Appendix A). The temporary advance is directly the result of the slumps. On the North side of the Head the cliffline has generally retreated at to m/year since 1938, generally averaging about m/year. In localised places there has been no retreat since 1938 owing to the presence of 10

15 4.3. FLOOD HAZARD relatively erosion resistant sandstone beds at cliffbase. At the Eastern extremity of Young Nicks Head, erosion rates are relatively low, averaging to m/year since 1886 (Table A-1, Appendix A). Appendix B includes a report by David Peacock, Asset Manager, Rivers and Land Drainage, GDC, which makes a significant contribution on this issue. The report identifies both tsunami and wave storms as the major causes of sea flooding. In the historic past low-lying land on the Muriwai coastal plains area has been extensively inundated by such flooding. In addition, the same low-lying land has also been inundated by fresh water runoff during intensive localised rainfall events such as occurred on 6 August 2002 and by floods in the Waipaoa River, for example:- 1938, 1944 and Much of the coastal plain between Muriwai Village and the Waipaoa River, and behind Orongo Beach, is generally less than 2.0 to 2.5 m a.msl. The low-lying area is a flood ponding basin. At Muriwai Beach Road and around the Wherowhero Creek there are low-lying access points for the sea to enter the flood ponding basin from the Wherowhero Estuary. Up until the mid 1970s, there was a similar low entry point at Orongo Beach and there is another entry point into this area between Muriwai Beach Road and the start of Young Nicks Head. In mid 1970s the Orongo Beach access point was artificially closed to the sea. The geologic trend of subsidence is a significant factor contributing to episodic flooding of the Muriwai coastal plain by lowering land levels Wave Storms Major wave storms to strike Poverty Bay include those that occurred in 1894, 1912, 1936, 1953, 1955, 1963, 1968, 1974, 1976, 1978, 1982, 1988 and The most significant events are likely to be those of September 1894, February 1936, February 1953, April 1968 and April These storms would have produced storm wave runup (SWRU) elevations of the order of 4 to 5 m a.msl which would have easily overtopped the Muriwai Spit and Orongo Beach dunes, penetrating to a greater or lesser degree into the flood ponding basin. Most of the wave storms noted approached from the South to Southeast quadrant. Only Orongo Beach would be exposed to such storms as Young Nicks Head provides partial protection for the Muriwai Beach area from full wave attack from this quadrant. Eyewitness accounts of the Wahine Storm (10 April 1968) indicate that the storm surge penetrated some 2 km inland at Orongo Beach but Mr Peacock was unable to establish ponding levels for this or any other wave storm events from the interviews. No records were found of the effects of the earlier storms which could well have been more severe. To have a major effect at Muriwai the wave storms would have to approach directly from the East quadrant as would be most likely during a Tropical Cyclone migrating in a South to Southeast direction. In the absence of historic observations a best approximation here of flood ponding levels from SWRU would be of the order of 2 to 3 m a.msl. 11

16 Tsunami Between 1840 and 1982, 6 tsunami events were recorded in the Gisborne region (de Lange and Healy 1986). The source areas for these tsunami were either distant or local. The earliest record of severe impacts is given by Hair (1985) who recorded that a tsunami about 1868 crashed over a Maori Village at Muriwai destroying houses and uprooting trees. The 1868 tsunami is thought to have originated from Chile. The 2 February 1931 Magnitude 7.8 Hawke s Bay Earthquake that nearly destroyed Napier generated a tsunami in Southern Poverty Bay which produced a big surge at Muriwai that washed some children over the top of the beach into a stream bed. The May 1960 Chilean tsunami produced a significant wave in the Wherowhero Estuary. Perhaps the best remembered tsunami was the locally generated event of 26 March 1947 following a Magnitude 5.9 Earthquake at 0833 hours that shook Gisborne District. The epicentre for the Earthquake was about 30 nautical miles offshore near the Hikurangi Trough. At about 0840 the coastline between Mahia Peninsula and Waipiro Bay was struck by 2 tsunami waves ranging from about 6 to 7 m at Mahanga Beach, to 3.7 m in Gisborne Harbour, to about 10 m at Wainui Beach (CMCL 1995; 2001; 2002). The 1947 event overtopped Muriwai Spit, surging across Wherowhero Estuary, filling the flood ponding area with a combination of seawater, fish and logs transported bodily inland off Muriwai Spit. Reliable eye witness observations were levelled by Mr Peacock to produce ponding heights ranging from 1.8 to 2.4 m a.msl, generally averaging 2.3 m a.msl. The state of the tide and sea at the time is not known. The source area for locally generated tsunami appears to be the Plate boundary between the Australian and Pacific Plates which are in constant motion. Clearly, the tectonically active area so close to Poverty Bay has a high probability of generating further tsunami of equal to or greater than the 1947 event. The tectonically subsiding Muriwai coastal plain is considered here to be significantly at risk of inundation from local tsunami compared to distantly generated tsunami which appear to be of lesser magnitude. 5. COASTAL HAZARD ZONE (CHZ) ASSESSMENT The CHZ incorporates a Coastal Landslip Hazard Zone (CLHZ), a Coastal Erosion Hazard Zone (CEHZ) for Orongo and Muriwai Beaches, and a Coastal Flood Hazard Zone (CFHZ) for the Muriwai coastal plain. The 50 Stations in Table A-1, Appendix A, provide the framework for CHZ calculations which are provided in Tables A-2 and A-3, Appendix A CLHZ ASSESSMENT The CLHZ applies to Young Nicks Head over a total length of coast of approximately 4.5 km and to the flanks of Orongo Beach over about 0.7 km. Equation 1 was adopted from CMCL (1995; 2001) to calculate the extent of the CLHZ, where: 12

17 CLHZ = [ S + R.T ] F Eqn [1] Where; S = Extent of seaward slope subject to landslip (m). R = Average rate of long-term (historic) retreat of the cliffbase (m/year). T = Hazard assessment period of 100 years. F = Safety Factor of 1.4 (40%). Factor S was determined from aerial surveys flown in 1938 and 2000 at scales of 1:11,600 and 1:5,000, respectively (Table A-2, Appendix A). For Factor R average long-term rates of seacliff retreat of m/year and m/year were adopted from Table A-1, Appendix A for the North and South facing seacliffs of Young Nicks Head, respectively, and m/year and m/year for the North and South headlands at Orongo Beach, respectively. For Factor F, a Safety Factor of 40% was adopted as it was judged that the uncertainties inherent In both Factors R and S were relatively more significant than the uncertainties for Northern Poverty Bay where a Safety Factor of 30% was adopted (CMCL 1995; 2001) CLHZ Widths Although CLHZ calculations are given for Stations 1 to 36 in Table A-2, Appendix A, the Stations were used as a guide to draw the CLHZ by hand on 1:5,000 Scale Orthophotomaps produced by the GIS Section of GDC. Initially, a line was drawn around the top edge of the landslide headwall or seaward ridge crest to define the extent of Factor S. The area so delineated is the Extreme Risk Landslip Zone (ERLZ). Next, a line paralleling the ERLZ was drawn to landward which was 25 m wide (R.T) on the North side of Young Nicks Head and 35 m wide on the South side. For Orongo Beach the line was 15 m wide on the North side and 20 m wide on the South side. The area so delineated is the Moderate Risk Landslip Zone (MRLZ). The MRLZ lies adjacent and generally parallel with the ERLZ. Finally, the Safety Buffer Zone (SBZ) was determined by multiplying the combined width of the ERLZ and MRLZ by 1.4 as measured directly on the orthophotomaps. The SBZ was drawn by making such calculations on the orthophotomap at every change point in width of the ERLZ + MRLZ. As might be expected, the SBZ width is proportional to the width of ERLZ + MRLZ. Around Young Nicks Head the ERLZ ranges in width from 35 to 270 m, and is widest on the South side where there are extensive bedding plane failures. At Orongo Beach the ERLZ ranges in width from 12 to 60 m. The MRLZ ranges in width from 25m on the North side of Young Nicks Head to 35 m on the South side where erosion rates are slightly greater. Similarly, at Orongo 13

18 Beach the MRLZ ranges in width from 15 m on the South side to 20 m on the North side, reflecting different erosion rates. Around Young Nicks Head the SBZ ranges in width from 24 to 122 m, and from 11 to 32 m at Orongo Beach. In total, the CLHZ ranges in width from 85 to 425 m at Young Nicks Head and 40 to 110 m at Orongo Beach (Table A-2, Appendix A). Within the CLHZ there is a high probability that assets will be damaged or destroyed this century by landslip CEHZ ASSESSMENT The CEHZ applies to the sand dunes of Orongo Beach over a total length of coast to approximately 0.35 km and to Muriwai Beach over about 4.3 km. Equation 2 was adopted from CMCL (2001) to calculate the extent of the CEHZ, where: CEHZ = [ (S max + D) + (X 50 + R) T 50 + (X R ) T 100 ] F Eqn [2] Where; R = Rate of long-term (historic) trend of net duneline advance, retreat or dynamic equilibrium (m/year). S max D X = Area subject to maximum short-term duneline fluctuation (m). = Area required for a vertical erosion scarp cut in a foredune of varying height to reach a stable gradient approximating the angle of repose of dry loose sand of = Rate of shore retreat (m/year) from local relative sea-level rise (RSLR) calculated by the Bruun Rule (Bruun 1962; 1983), where: X = la_ h + d Eqn [3] And; a 50 = Rate of local RSLR up to 2050 (m/year). a 100 = Rate of local RSLR up to 2100 (m/year). d = Average height of foredune crest above MSL (m). h = Average closure depth below MSL (m). l = Average distance from the foredune crest to the average closure depth (m). F = In accordance with CMCL (1995) a Safety Factor of 1.3 (30%) was adopted to accommodate uncertainties in Factors R, X, S and D. 14

19 Quantifying the Factors The parameters for the above factors in Equations 2 and 3 were quantified in Table A-3, Appendix A, for each Station as follows: R : Long-term rates were averaged from the most reliable rate data in Table A-1, Appendix A, the averages ranging from accretion at 0.23 m/year at Orongo Beach, through to 1.26 m/year accretion along Northern Muriwai Beach and m/year erosion along the South. S : Maximum short-term duneline fluctuations were quantified from the combinations of erosion-accretion patterns revealed by comparing historic shoreline positions, field observations and sequential vertical aerial photography, and ranged from a minimum of 25 m at Orongo Beach up to a maximum of 300 m at Muriwai Beach. T : To quantify the risk zones, hazard assessment periods of 50 and 100 years were adopted. D : As the foredunes seldom exceeded 2 m a.msl and little accurate levels were available for this study the dune stability factor was combined with Factor S as part of the overall short-term fluctuation measured for S (S + D). a : Factor R used in Equation 2 automatically incorporates the effects of historic local RSLR. Subtracting historic Local RSLR from the latest IPCC (2001) most likely projection to derive Factor a [Eqn 3] avoids double counting the effects of SLR in the CEHZ assessment. Historic local RSLR was calculated by adding the latest average longterm rise in sea level around New Zealand last century of 1.6 mm/year (Hannah 2003) to the Late Holocene subsidence rate of 2.5 mm/year for Northern Muriwai, diminishing linearly South to an assumed subsidence rate of 0.0 mm/year for the Orongo Beach-Young Nicks Head area, where there is no known evidence of subsidence. Rounding to the nearest 0.5 mm provided net historic RSLR values of 4.0 mm/year for North Muriwai to 1.5 mm/year at Orongo Beach. The most likely mid-range projections by the IPCC (2001) above 1990 levels are mm ( mm/year) by 2050 and mm ( mm/year) by 2100, averaging 400 mm (3.64 mm/year). For 2050, residual rates of RSLR to quantify Factor a range from zero in the South to, 1.5 mm/year in the North. For 2100, RSLR values range from 0.5 mm/year in the South to 3.0 mm/year in the North. d : A closure depth of -10 m was adopted from Gibb (1995; 2001) and identified as a contour on Royal NZ Navy Charts NZ5613 and NZ55 at scales of 1:24,000 and 1:200,000, respectively. l : Was measured on the RNZN Charts, ranging from 960 m to 2040 m at Muriwai Beach, to 1,400 m at Orongo Beach. 15

20 h : Was adopted from East Cape Catchment Board Sheet 4257-E, 1:10,000 Scale, Cyclone Bola Floodspread Map, ranging from 1.5 to 4.0 m a.msl at Muriwai Beach to about 2 m a.msl at Orongo Beach CEHZ Widths Although CEHZ calculations are given for Orongo Beach (Stations 4-6) and for Muriwai Beach (Stations 37-50) in Table A-3, Appendix A, the Stations were used as a guide to draw the CEHZ by hand on the 1:5,000 Scale Orthophotomaps produced by the GIS Section of GDC. Initially, the EREZ was drawn with respect to the 2002 duneline position. Along Muriwai Spit, the EREZ includes the entire spit between Poverty Bay and the Wherowhero Estuary because of its well-documented history of instability, ranging in width from 80 to 200 m. East of the spit the EREZ includes the coastal plain subject to erosion from the migration of the Wherowhero Estuary mouth and is 100 m in width. At Orongo Beach, the EREZ ranges in width from 25 to 40 m to allow for short-term duneline fluctuations (Table A-3, Appendix A). The HREZ only applies to the small beach East of the Wherowhero Estuary and is 40 m in width. Here, the long-term average erosion rate of m/year will be enhanced to m/year by 2050 by forecast SLR. In contrast, at Orongo Beach, the historic rate of accretion of 0.23 m/year will effectively nullify potential erosion of m/year from SLR. Along Muriwai Spit, the HREZ would extend into the Wherowhero Estuary so it is not appropriate to plot it (Table A-3, Appendix A). The MREZ does not apply to Muriwai Spit for the same reasons as the HREZ. It applies to Orongo Beach where forecast erosion of m/year by 2100 from SLR will overwhelm long-term accretion at 0.23 m/year. The MREZ is 10 to 15 m wide at Orongo Beach. For the beach East of the Wherowhero Estuary, the MREZ is 70 m wide as a result of forecast erosion of m/year from SLR by 2100 enhancing the historic erosion rate of -0.6 m/year to 1.08 m/year (Table A-3, Appendix A). As might be expected, the SBZ applies to the CEHZ s in all three areas. Along Muriwai Spit the SBZ extends partly into the Wherowhero Estuary but has been adjusted to include washover fans from past wave overtopping and ranges in width from 20 m to 90 m. For the beach East of the Wherowhero Estuary, the SBZ is 60 m wide and for Orongo Beach it is 10 to 15 m (Table A-3, Appendix A). In total, the CEHZ ranges in width from about 90 m to 390 m along Muriwai Spit, 50 m to 70 m at Orongo Beach, and is uniform 270 m along the beach East of the Wherowhero Estuary mouth (Table A-3, Appendix A). Within the CEHZ there is a high probability that assets will be damaged or destroyed this century by sea erosion. 16

21 5.3. CFHZ ASSESSMENT The CFHZ applies to the Muriwai coastal plain including the valley flood behind Orongo Beach. The CFHZ includes two important interconnected components; a flood velocity zone, and; a flood ponding zone Flood Velocity Zone The Flood Velocity Zone (FVZ) is the area of land adjacent to Poverty Bay that is subject to wave overtopping during severe wave storms and tsunami. Based on observations of past events, the FVZ includes the Muriwai sand spit, the Orongo Beach foredune, the mouth area of Wherowhero Creek, and the mouth areas of other low-lying areas bordering the Wherowhero Estuary, that act as conduits for wave bores to penetrate into the flood ponding basin. Also included are low-lying areas adjacent to the Karaua Stream mouth and true right bank of the Waipaoa River below the Railway Bridge Flood Ponding Zone The Flood Ponding Zone (FPZ) is the area of land adjacent to the FVZ that is subject to temporary inundation following a severe wave storm or tsunami. To determine the extent of the FPZ it is first necessary to assess a design sea flood elevation above MSL Gisborne Datum, that is likely to be reached this century. Second, it is necessary to define the extent of the FPZ by precisely surveying the design sea flood elevation. The design sea flood elevation was assessed as follows: Elevation a.msl reached by the 1947 tsunami Projected average most likely global SLR by 2100 Safety Factor DESIGN SEA FLOOD ELEVATION 2.3 m 0.40 m 0.30 m 3.00 m a.msl The Safety Factor of 0.30 m was determined by the standard Root-Sum- Square method (Gibb 1986; Berryman et al. 2000) from uncertainties of 0.25 m subsidence, global SLR exceeding the average value by 0.1 m, and the 1947 tsunami elevation being exceeded by 0.15 m by similar events, as follows: RSS (2100) = (0.25) 2 + (0.15) 2 + (0.10) 2 = m Rounded = 0.30 m 17

22 CFHZ The CFHZ is relatively extensive, and includes the entire seaward part of the Muriwai coastal plain where land elevations range from 1.0 m to 3.0 m a.msl. At Orongo Beach the CFHZ includes the entire valley, extending inland about 1.5 km. Most of Muriwai Village is landward of the CFHZ which extends inland about 1.1 km along Muriwai Beach Road. The CFHZ increases in width Northward from about 1.5 km adjacent to the Wherowhero Creek mouth to about 2.5 km adjacent to the Waipaoa River mouth. There is insufficient information to define the full extent of the FVZ except to say that it includes all of Muriwai Spit, the sand dunes at Orongo Beach, extending an unknown distance inland from the Wherowhero Estuary and up the Orongo valley. Within the FVZ there is a high probability that assets will be damaged or destroyed this century by wave bores. Within the FPZ there is a high probability that assets located on land below 3.0 m a.msl, will be damaged this century by temporary sea water inundation. 6. SUMMARY AND CONCLUSIONS 1. In relative terms, the Southern Poverty Bay area is one of the most naturally hazardous coastal areas within Gisborne District. The 10 km-long coast is subject and will continue to be subject to marine erosion and flooding, and coastal landslip. 2. The CFHZ is composed of a Flood Velocity Zone (FVZ) and a Flood Ponding Zone (FPZ). The FVZ includes the Muriwai sand spit and Orongo Beach foredune that are subject to wave overtopping during severe wave storms and tsunami. The FPV is the area of land on the subsiding Muriwai coastal plaint hat is subject to temporary inundation to a ponding level of 3.0 m a.msl from wave penetration during tsunami and exceptional wave storms. The CFHZ extends inland from about 1.1 km by Muriwai Village (which is above 3 m MSL) to about 2.5 km by the Waipaoa River mouth. 3. The CLHZ is composed of an Extreme Risk Landslip Zone (ERLZ), a Moderate Risk Landslip Zone (MRLZ), and a Safety Buffer Zone (SBZ), which encompass the seacliffs of Young Nicks Head and the promontories adjacent to Orongo Beach. On the North facing coast the retreating seacliffs are subject to rockfalls and on the South side they are subject to extensive relatively large scale bedding plane failures. In total, the CLHZ ranges in width from 85 m to 425 m around Young Nicks Head and 40 m to 110 m at Orongo Beach. 4. The CEHZ is composed of an Extreme Risk Erosion Zone (EREZ), a High Risk Erosion Zone (HREZ), a Moderate Risk Erosion Zone (MREZ), and a Safety Buffer Zone (SBZ), which encompass Muriwai and Orongo Beaches. Current erosion rates will be enhanced by erosion from sea-level rise (SLR) from global warming from an enhanced Greenhouse Effect. In total, the CEHZ ranges in width from about 90 m to 390 m along Muriwai Spit, 50 m to 70 m at Orongo Beach, and is a uniform 270 m wide along the beach East of the Wherowhero Estuary mouth. 5. Council should promote land uses that are appropriate for land subject to natural hazards this century within the 2004 Southern Poverty Bay Coastal Hazard Zones (CHZ). There is 18

23 a reasonably high probability that assets located within the CHZs will be damaged or destroyed this century. 7. RECOMMENDATIONS It is recommended that Gisborne District Council, after due consideration of this report and accompanying Coastal Hazard Maps: i. ADOPT the 2004 Southern Poverty Bay Coastal Hazard Zone between Orongo Beach and Waipaoa River mouth for the purposes of controlling coastal subdivision, use and development and alerting the public of the risks to coastal property from the identified natural coastal hazards of marine erosion and flooding, and coastal landslip. ii. INCORPORATE the 2004 Southern Poverty Bay Coastal Hazard Zone into Council s Proposed Regional Coastal Environment Plan and Proposed Gisborne District combined Regional Land and District Plan to replace the 1994 Area Sensitive to Coastal Hazards. iii. PROVIDE for public seminars and open days at Muriwai Village to disseminate both the findings of this study and Council s decisions with respect to managing subdivision, use, and development within the 2004 Southern Poverty Bay Coastal Hazard Zone. iv. MONITOR and quantify the identified coastal hazards of marine erosion and flooding, and coastal landslip, to the extent that the data collected by Council can be utilised from time to time to review the 2004 Southern Poverty Bay Coastal Hazard Zone. v. REVIEW the 2004 Southern Poverty Bay Coastal Hazard Zone using similar methods either once every 10 years, OR after occurrence of significant hazardous events such as large storms, tsunami, floods, or large earthquakes, OR after significant changes in global climate change forecasts including potential sea-level rise, by the Intergovernmental Panel on Climate Change. 8. REFERENCES Berryman, K., Marden, M., Eden, D., Mazengarb, C., Ota, Y. & Moriya, I. 2000: Tectonic and paleoclimatic significance of Quaternary river terraces of the Waipaoa River, east coast, North Island, New Zealand. NZ Journal of Geology & Geophysics. 43: Brown, L.J.H. 1995: Holocene shoreline depositional processes at Poverty Bay, a tectonically active area, northeastern North Island, New Zealand. Quaternary International 26: CMCL 1994: Initial assessment of Areas Sensitive to Coastal Hazards for selected parts of the Gisborne District Coast. 2nd Report C.R. 94/18, prepared for Gisborne District Council. December p. + Appendices 1 & 2. CMCL 1995: Assessment of Coastal Hazard Zones for northern Poverty Bay and Wainui Beach, Gisborne District. Report C.R. 95/3, prepared for Gisborne District Council. July p. 19

24 CMCL 2001: Review of the 1995 Wainui Beach Coastal Hazard Zone. Report C.R. 2001/6, prepared for Gisborne District Council, November p + 3 Appendices. CMCL 2002: Coastal Hazard Zone Assessment for Mahanga Beach, Wairoa District. Consultancy Report C.R. 2002/5, prepared for Mahanga Beach Ltd. November p De Lange, W.P. & Healy, T.R. 1986: New Zealand Tsunamis New Zealand Journal of Geology and Geophysics, Vol. 29: Gibb, J.G., 1986: A New Zealand regional Holocene eustatic sea-level curve and its application to determination of vertical tectonic movements. A contribution to IGCP-Project 200. Proceedings of the International Symposium on Recent Crustal Movements of the Pacific Region. Wellington, New Zealand February Royal Society of New Zealand Bulletin 24: Gibb, J.G., Sheffield, A., and Foster, G. 1992: A standardised Coastal Sensitivity Index based on an initial framework for physical coastal hazards information. Department of Conservation Science and Research Series. No p. Hair, Angela. 1985: Muriwai and Beyond. (Publisher unknown). Hannah, J. 2003: Long-term Sea Level Change in NZ An Updated Analysis. Coastal News Issue 22, April IPENZ: p4. IPCC. 2001: Climate Change 2001: The Scientific Basis. Summary for Policymakers and Technical Summary of the Working Group I Report. Part of the Working Group I contribution to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK. See also web site: Mazengarb, C. & Speden, I.G. (compilers) 2000: Geology of the Raukumara area. Institute of Geological & Nuclear Sciences 1:250,000 Geological Map 6. 1 Sheet + 60p. Varnes, D.J. 1984: Landslide hazard zonation: a review of principles and practice. UNESCO. Special Report, Paris. 63p. 20